Integrated circuits typically comprise a large number of active and passive electronic devices formed on a single semiconductor wafer. Electrical connections (or ‘bonds’) between the wafer and external electrodes of (say) a wafer package are typically made by bonding ‘bondwires’ to ‘bondpads’ provided on the wafer.
The problem exists that the surface of a bondpad can become contaminated. If the amount of contamination exceeds a critical value, bonds formed between a bondwire and a bondpad may be unreliable and susceptible to breakage. In some cases, if the amount of contamination is excessive, it may be difficult to form a bond between a bondwire and a bondpad.
Bondpads are typically formed from a metallic material such as aluminum. Contamination of the surface of an aluminum bondpad by fluorine (F) can result in the formation of aluminum-fluoride-oxide (AlxFyOz) due to reaction between fluorine, aluminum, oxygen and moisture over time. During storage of the wafer, the amount of AlxFyOz, can increase until the formation of reliable bonds to a bondpad is no longer possible.
It is known to attempt to determine a shelf lifetime of a wafer provided with bondpads in order to reduce a failure rate of packaged integrated circuits by performing a ‘shelf lifetime test’. The shelf lifetime of a wafer is the length of time for which the wafer may be stored under suitable, predetermined storage conditions before its condition is no longer acceptable for subsequent IC fabrication.
Knowledge of the shelf lifetime is important because it enables IC manufacturers to reduce the number of IC devices of inferior quality. It also enables wafer suppliers to provide a warranty or guidance to a manufacturer in respect of the shelf lifetime of a wafer.
The traditional ‘shelf lifetime test’ involves the steps of placing a wafer from a given batch of wafers in a wafer box that has a small amount of deionised (DI) water in a lower portion of the box. The wafer box is closed and stored at room temperature for a predetermined period of time. The selected wafer is known as the ‘test wafer’.
The test wafer is inspected after a test period corresponding to an effective storage period under normal storage conditions of one year (or any other required period of time) from the date of fabrication of the bondpads. An amount of contamination on the bondpads of the test wafer is determined; this determination may be made by reference to one or more bondpads of the test wafer.
If the bondpads of the test wafer are found to have a contamination level below a critical contamination level for bond formation, the batch of wafers from which the test wafer was taken may be considered suitable for storage for a predetermined time, for example one year from the date of fabrication of the bondpads as discussed above.
If the bondpads of the test wafer are found to have a contamination level equal to or above the critical contamination level for bond formation, measures may be taken to reject the batch of wafers from which the test wafer was taken. Such wafers might then be discarded, or subjected to a cleaning process before subsequent bond formation.
Empirical results suggest that, in order to simulate a storage period of 1 year under normal storage conditions by means of the test described above, a period of testing of from about 23 to about 198 days is required. This period is calculated assuming that the wafer is stored at a temperature of 22° C. in an environment having a relative humidity of 75%.
The test period of 23 to about 198 days is excessive in the context of IC fabrication. The requirement to perform a shelf lifetime test on a test wafer of a batch of wafers before shipping the batch to a customer introduces a significant delay in the delivery of the batch.
Furthermore, the results of the traditional test are found to be unrepeatable in a reliable manner. This introduces a level of uncertainty in the determination whether a given batch of wafers may be warrantied as suitable for being stored for a give period of time.